Many methods have been proposed to produce an air-permeable film by stretching, uniaxially or biaxially, a film made of a composition comprising a polyolefin resin and an inorganic filler, thereby introducing a number of open voids into the stretched film. It has been started to apply such air-permeable films to sanitary materials such as disposable diapers and sanitary napkins.
This air-permeable film, however, possesses stiffness due to molecular orientation of the polyolefin resin by stretching. Thus, it is unsuitable for use in applications requiring soft touch like cloth because of its stiffness.
In order to provide the air-permeable film with soft touch, it may be considered to use a resin composition which is of low stiffness, i.e., soft. For example, a method of compounding a liquid polymer has been proposed. In accordance with this method, however, an air-permeable film having satisfactory soft touch has not yet been obtained, but the film becomes excessively adhesive.
It is also considered effective to control a degree of orientation by decreasing a stretching ratio, and to decrease the thickness of the film. When, however, a film made of a composition comprising a polyolefin resin and an inorganic filler is stretched to produce an air-permeable film, if the stretching ratio is decreased, local necking occurring at the start of stretching remains in the stretched film product and thus the stretched film is of low thickness evenness. This tendency becomes more marked when the thickness of the film is decreased.
In accordance with the conventional method of producing an air-permeable film by stretching a film made of a composition comprising a polyolefin resin and an inorganic filler, an air-permeable film having soft touch and a decreased and uniform thickness has not yet been produced particularly at a low stretching ratio.